It is a constant endeavor to find improved ways to apply resins to encapsulate components, chips and/or modules. Presently chip or glob top encapsulation is generally accomplished with premixed materials. Pitfalls associated with this method range from improper resin storage, difficulty of flow control, and improper tube or syringe properties for application requirements. In order to extend the mixture's shelf life, it has to be frozen and subsequently thawed for use. It is absolutely essential to maintain the mixture at its required temperature. Failure to maintain proper storage temperature causes accelerated aging. This often results in increased material viscosity requiring ejection at a high pressure.
Most encapsulation methods presently employed require long cure cycles. A long cure cycle is particularly costly and inappropriate for use in an automated assembly line for a high volume throughput. This is evident by considering a typical assembly line situation. An assembly line with a line speed of 12 feet/min, and a cure time at raised temperature of 10 minutes, requires an oven at least 120 feet long. Currently, proper curing of a one pot, premixed encapsulation material takes about two hours at a raised temperature. To partially overcome this problem for devices mounted on a continuous tape-like strip, the parts are rolled after the encapsulating material, the encapsulant, has gelled. This is to prevent the parts from flowing, peeling or sticking to each other. In addition, the parts have to be cut and placed in a cassette for post curing.
Successful use of a one pot reactant material mixture in a large volume encapsulation application is difficult. In any one pot mixture, a reaction is progressing at all times. Increasing the temperature of the reacting system accelerates the reaction, while cooling slows the reaction. The actual change in the reaction rate is dependent on the viscosity. Intrinsically it depends upon the activation energy for the reaction. The reaction rate approximate doubles with every 10.degree. Centigrade increase in temperature. The use of very high temperatures to force a reaction to proceed extremely quickly is limited by the thermal stability of the reactants and products of the reaction. For aliphatic containing epoxy resins, this temperature is limited to about 170.degree. C. Aromatic epoxy resins can sustain substantially higher temperatures, but the reactivity of these species is substantially slower than for aliphatic systems and some reaction chemistries are not suitable for aromatic based resins.
One solution is to include a very efficient and fast catalyst system in the filled epoxy resin used for chip encapsulation. Shelf life becomes a very significant problem if the catalyst is very fast. Methods and devices to mix two liquids with and without the simultaneous application of thermal energy exist. Unfortunately, none can be successfully used in an application requiring very fast reaction. The problem of uniform heating throughout the mixing chamber was addressed by Griffith, U.S. Pat. No. 4,678,881, by using a heated paddle. The present invention presents a solution in which the catalyst is mixed with the resin only immediately prior to its being dispensed to encapsulate a device.
Inoue, U.S. Pat. No. 4,834,545, addresses the problem of mixing and dispensing an admixture of two materials by utilizing a conical mixing chamber and axial motion of the mixing spindle to eject the resin. There is no provision for heating. If the entire chamber was heated, residual resin in the mixing chamber following ejection would cure and result in buildup and eventually clogging and poor dispense column control.
Continuous reactors, such as that presented by Wilt, U.S. Pat. No. 4,438,074, typically have very long residence times with temperature control. There is no means of dispensing controlled amounts of the product and certainly no means for heating the mixture after dispense. These systems are typically used in mixtures which include a solvent. The resultant lower viscosity mixture does not require large shearing forces to be dispensed from the mixer. Materials requiring relatively large shearing forces preclude many means for dispensing the resin. The present invention is applicable even to filled materials which generally may require large shearing forces to be dispensed.
Haeuser, U.S. Pat. No. 4,741,623, shows a means for pumping a liquid into a mixing chamber, but gives no indication as to how the reactants are mixed or dispensed from the mixing chamber or subsequently heated.
Japanese patent, JP7833275, describes the mixing of two reactants and injection into a mold. This patent does not concern itself with controlled dispense or heating the reactants upon exit. The present invention used in conjunction with the device described in this patent would result in the resin curing within the mixing chamber and clogging it.
It is an object of the present invention to provide a means that can satisfactorily deliver a controlled volume of fast curing resin in a very precise and consistent manner around a device. This would eliminate the need for very long and expensive tunnel ovens in assembly line applications. It is another object to use epoxy resins that are highly filled to attempt to better match the coefficients of thermal expansion (CTE) of the resin and the chip. A very heavy loading of greater than 50 volume percent decreases the mixture's CTE to about 30 ppm as compared to about 60-80 ppm for the neat resin. The chip's CTE is about 2 ppm. The CTE of the lead frame on which the chip is mounted is about 18-25 ppm. By reducing the CTE mismatch, residual stress is reduced in the resulting structure.
It is another object of the present invention to reduce the resin's viscosity by heating it prior to its being applied to the chip. This allows the resin to settle around the chip and to effectively encapsulate the chip without leaving substantial voids or bubbles. It also reduces the time necessary to reheat the resin on the chip thereby reducing the necessary curing time. It enables the reaction to start prior to deposition on the chip. This is advantageous since viscosity increases very slowly in the initial stages of a reaction. The rate of increase of the viscosity increases with the extent of reaction until the gel point is reached. At this point the viscosity is essentially infinite. Prior heating also helps the small amount of resin to drop properly from the tip of the applicator. Proper resin dropping is related to surface tension and viscosity which decrease with temperature.
If the mixer or the stirrer in the mixer is heated in entirety, the chemistry starts prematurely. Premature heating would cause the formation of surface coatings even if the resin is flushed out of the mixer often. The coatings gel and become permanent within the mixer. Ultimately, the thickness of this coating becomes sufficient to contaminate the mixer. This would require that the mixer be disassembled and cleaned or replaced.
The present invention overcomes these problems and provides a method and apparatus which may be employed in extremely high volume throughput applications. The encapsulating material is mixed in-situ which leads to a shortened cure time in the order of several minutes or less, for example five minutes. This process also provides improved and increased processing capability and longer material pot life.